This application is based on the European Patent Application No. 97830674.4, the content of which is incorporated hereinto by reference.
The present invention relates to an optical code scanning reader with laser beams travelling along a minimum of two different optical paths and focused to read said optical code at different distances.
Patent EP-0 480 348 describes a bar code reader comprising a minimum of two laser beam sources and suitable optical means for guiding the emitted beams onto a reading area and receiving the light diffused from said area. The optical means focus each beam independently on different areas of said reading area, thus enabling an optical code to be read at different reading distances.
Each of the laser beams is capable of "resolving" a portion of a bar code having a minimum assigned dimension in a given area around the focal point and said area determines the depth of field of the reader for said portion of code.
The above-mentioned reader uses two laser beams focused on two different points. One beam, dedicated to reading the optical code from a close distance, is focused on a point closer to the optical receiving means than the focal point of the other beam which is dedicated to reading from a remote distance. Two reading fields are thus obtained which, although partially overlapping, create a total depth of field greater than that of a single laser beam.
In the reader in accordance with EP-0 480 348 the laser beams travel along "coincident" optical paths and the individual optical paths are caused to overlap by the use of deflector elements consisting of beamsplitters. The beamsplitters, however, reduce the utilisable power of each individual laser beam by 50%. Where several laser beams are present, therefore, loss of power increases in proportion to the number of beamsplitters used, reducing the available power by 50% for each additional beamsplitter.
The reader described in the above-mentioned patent could hypothetically be improved by reducing the loss of available power by means of polarized beamsplitters, since it is well-known that these are more efficient.
However, if wishing to use more than two lasers, the subsequent beamsplitters would in any case have to be of non-polarized type and this would, as already mentioned, lead to a loss of power of 50% for each conventional type of beamsplitter added.
Polarized beamsplitters could moreover not be used when optical codes printed on shiny surfaces have to be read. In such cases it is in fact necessary for the vector of polarization, by means of which the laser beams are guided onto the code, to be identically orientated.
The inventors took into consideration the possibility of causing the different laser beams to travel along different optical paths, but this gives rise to the problem of parallax. In fact, except for one laser beam, all the other beams are offset in respect to the optical axis of the optical receiving means, i.e. said other beams occupy a spatial position remote from the optical axis. Since offset between laser beam and optical receiving means causes a shift of the image of the optical code which is formed on a photoreceiver (the parallax phenomenon), it is possible for a significant part of the light diffused by the optical code to be directed outside the sensitive area of the photoreceiver.
The inventors have, however, found that this problem can be minimised by suitably offsetting the laser beams in such a way that their distance from the optical axis of said optical receiving means is greater for the laser beam which reads the optical code at the maximum distance and is gradually reduced for the laser beams which read the optical code at gradually decreasing distances.
The inventors have also found that optimum reading of the optical code can be obtained by selection of a minimum reading distance such that at least a prefixed quantity of energy of the light diffused by said optical code falls within the sensitive area of the photoreceiver.
In accordance with the present invention, an optical code scanning reader comprising means for generating laser beams, a least one optical receiving means for collecting and focusing of the light diffused by said optical code, said optical receiving means having an its own optical axis, and a photoreceiver element associated with said optical receiving means, said photoreceiver having a preselected photosensitive area, characterized in that said laser beams travel along a minimum of two different optical paths, in that at least one of said optical paths is offset in respect to said optical axis and that the value of said offset (H.sub.o) is preselected such that the quantity of energy of light diffused by said code which reaches said photosensitive area for each beam attains a preselected value.